1. Field of the Invention
The present invention relates to a liquid crystal display apparatus where an electric signal is applied to each display electrode via a nonlinear element, and a method for fabricating the same.
2. Description of the Related Art
In recent years, in order to respond to the requests for increasing the contrast and the display capacity of liquid crystal display apparatus, liquid crystal display apparatus having various types of active elements have been proposed. Among them, an apparatus incorporating nonlinear elements having a structure of metal-insulator-metal (MIM) (hereinafter, such an element is referred to as a MIM element) has attracted attention because the fabrication process thereof is comparatively simple.
A typical MIM element is composed of a lower electrode made of tantalum (Ta) formed on a glass substrate and an upper electrode made of titanium (Ti), chromium (Cr), and aluminum (Al) formed on the lower electrode via an insulating film made of tantalum pentaoxide (Ta.sub.2 O.sub.5). The upper and lower electrodes are desirably formed of a same material in consideration of the symmetry of the current-voltage (I-V) characteristic of the MIM element, i.e., the positive-negative symmetry when a current flowing from one electrode to the other is referred to as a positive current. When a same material is used for the upper and lower electrodes, however, the lower electrodes may become corroded at the patterning of the upper electrodes. Accordingly, photolithography and etching can not be used for the patterning of the upper electrodes. This is why the above-mentioned materials are used for the formation of the upper electrodes. Using these materials, the lower electrodes will not be corroded, and the positive-negative symmetry of the I-V characteristic of the MIM elements will not be significantly diminished.
A known liquid crystal display apparatus using such MIM elements as switching elements is disclosed in Japanese Patent Publication No. 5-10652. Referring to FIGS. 14 to 20, this conventional liquid crystal display apparatus will be described.
FIGS. 14 and 15 are a plan view and a sectional view, respectively, of the portion of a substrate 41 where a plurality of MIM elements are formed (hereinafter, such a substrate is referred to as an element-formed substrate), corresponding to one MIM element. FIG. 16 is a partial plan view of the element-formed substrate 41. The element-formed substrate 41 includes a glass substrate 42 and signal lines 43, MIM elements 47, and pixel electrodes 48 formed on the glass substrate 42, As shown in FIG. 14, each signal line 43, which is made of Ta, has a branch 44 which serves as a lower electrode of each MIM element 47. An insulating film 45 is formed on the signal line 43 or the lower electrode 44. The insulating film 45 is made of Ta.sub.2 O.sub.5 formed by anodization. An upper electrode 46 made of Ti is formed on the insulating film 45. As shown in FIGS. 14 and 15, the ends of the upper electrode 46 are electrically connected to the corresponding pixel electrode 4a made of ITO (indium-tin-oxide) and the like.
Further, as shown in FIG. 15, island portions 50 can be formed near the lower electrode 44 constituting the MIM element 47 simultaneously with the formation of the signal line 43 and the lower electrode 44 in the same step, so as to ensure the electrical connection between the lower electrode 44 and the pixel electrode 48.
As shown in FIG. 17, a counter substrate 51 located parallel to the element-formed substrate 41 includes a glass substrate 52 and a plurality of counter electrodes 53 formed in a stripe on the glass substrate 52. The element-formed substrate 41 and the counter substrate 51 are bonded together so that the geometric planes of the signal lines 43 and the counter electrodes 53 are perpendicular to each other. Liquid crystal is then injected into a space between the substrates 41 and 51. Thus, a liquid crystal display apparatus as shown in FIG. 18 is fabricated.
At the operation of pixels of the liquid crystal display apparatus, the portions of liquid crystal molecules except for those present above the pixel electrodes 48 have no contribution to display. Further, since light leaks through the gaps between the pixel electrodes 48, the contrast of display may lower, especially in the case of the normally white mode. For the above reasons, a black organic polymer film is formed by printing or a metal or metal oxide film is formed by photolithography as a black mask on the counter substrate 51, at positions corresponding to the gaps between the pixel electrodes 48 when the counter substrate 51 and the element-formed substrate 41 are bonded together.
In order to achieve high-grade and high-precision display by the liquid crystal display apparatus having the above structure, it is necessary that the capacity of the MIM element 47 be as small as possible while the capacity of the liquid crystal be sufficiently large compared with the capacity of the MIM element 47, In order to reduce the capacity of the MIM element 47, the size thereof can be reduced. However, the size of the MIM element 47 is restricted by an exposing apparatus used in the fabrication process and an etching method employed. Moreover, it is difficult to stably form the components of the MIM elements 47 with a width of several microns by patterning. As a result, the lower electrode 44 (i.e., the branch of the signal line 43) becomes narrower at the end as shown in FIG. 14 when it is formed by patterning by photolithography. Therefore, the width of the portion of the lower electrode 44 which is to constitute the MIM element 47 together with the insulating film 45 and the upper electrode 46 is unstable. This results in a variation in the size of the MIM elements 47 and thus a variation in the capacitance thereof. An error therefore arises in the ratio of the capacitance of the MIM element 47 to that of the liquid crystal, generating display variation and thus significantly reducing production yield.
In order to achieve high-contrast display, the area of the display plane except for the portions used as pixels to be driven needs to be as small as possible, so as to effectively use light incident to the liquid crystal display apparatus. In a transparent-type normally-white mode liquid crystal display apparatus, a black mask is required to prevent light from leaking through gaps between the pixel electrodes 48 when the pixels are being driven and thus to prevent contrast from lowering. The formation of a black mask, however, increases the number of fabrication steps, resulting in cost increase.
The upper electrode 46 of the MIM element 47 also becomes narrower at the end, as in the case of the lower electrode 44. The pattern of the upper electrodes 46 is finer as compared with the area of the glass substrate 42. The density of the pattern to be used for etching is therefore low (i.e., the portion to be etched per unit area is large). Thus, the etching rate becomes high, resulting in over-etching where etching proceeds excessively beyond a predetermined pattern (shown by the dotted line in FIG. 19). The over-etching gradually proceeds from the corners of the pattern and finally reaches the portion thereof where the lower electrode 44 is formed underneath. Thus, the width of the portion of the upper electrode 46 located above the lower electrode 44 is also unstable. This results in a variation in the capacitance of the MIM elements 47, as in the case of the lower electrode 44. An error arises in the ratio of the capacitance of the MIM element 47 to that of the liquid crystal, generating display variation and thus significantly reducing production yield.
The above trouble of the unstable width of the upper electrode 46 arises whichever material, Al, Cr, or Ti is used for the upper electrode. This trouble is especially significant when Ti is used. When Al is used for the upper electrode, the positive-negative symmetry of the I-V characteristic, the non-linearity, and the contrast of the resultant liquid crystal display apparatus are low, compared with when Cr or Ti is used. When Cr is used for the upper electrode, a harmful ionic substance such as hexavalent chromium is generated at the etching. This requires additional steps and facilities for treating such a harmful substance, resulting in cost increase.